Polyurethanes are typically manufactured as a reaction product of a polyisocyanate and a polyol. Other reactive compounds such as water, glycol, or diamines may be added to impart specific handling or mechanical properties. Polyurethane properties may also be modified with additives such as carboxylic acids, catalysts, solvents, surfactants, blowing agents, stabilizers, colorants, fillers, viscosity modifiers, release agents and plasticizers. The polyols generally used in manufacturing polyurethanes are low molecular weight polymers such as polyethers, polyesters, polycarbonates, polyacrylics, melamine and polybutadiene polyols. Such polyols are generally provided with at least two (end terminal) hydroxyl groups and may have a low level of residual acid functionality acquired by use of carboxylic acids during the manufacture of the polyols which incompletely react with diols to form the polyol. For example, in manufacturing polyester polyols, the reactant carboxylic acids may provide a residual acid value of less than 10 mg KOH/gram, with the majority of polyester polyols having acid values less than 1.5. Other polyols generally have an acid value less than 1.
There have been attempts to provide acid functionality to such polyols for use in forming water borne polyurethanes. One method to provide limited acid functionality to polyols includes use of carboxylic acid anhydrides. The anhydride group reacts readily with a hydroxyl group in the polyol to form a single molecule joined by an ester linkage and leaving a carboxylic acid group in the molecule. However, in many circumstances, the acid group can react with another hydroxyl group to yield a second ester which is an undesired side reaction.
One method of providing acid functionality, by incorporating carboxylic acid groups into the polymer backbone, includes use of dimethylolpropionic acid (DMPA). DMPA has a molecular weight of 136, two hydroxyl groups and one carboxylic acid group.
Generally, to form the polyurethane, the DMPA is reacted with the starting polyol and a diisocyanate to form an isocyanate-terminated prepolymer. The prepolymer is made at a temperature which allows the hydroxyl groups to react with excess isocyanate without consuming acid groups. The acid functional prepolymer becomes a water borne polyurethane dispersion by neutralizing the acid groups, dispersing in water and curing with a diamine. Difficulties which arise from use of DMPA are that it is a high melting solid material with limited solubility in polyols. As such DMPA is typically pre-dissolved in the polyol with solvent at temperatures over 100.degree. C. Such a step adds processing time and cost to polyurethane manufacture and increases the need for organic solvents. In polyurethanes, the urethane linkages give strong hydrogen bonding and usually phase separate into what is known as a hard segment. However, when using DMPA, the final polyurethanes have an acid group within a few carbons of the urethane linkages which interferes with hard segment formation by inhibiting phase separation. As a result, mechanical properties of the polyurethane suffer.
In an attempt to improve upon the DMPA technology, products were developed which react the same starting polyols with trimellitic anhydride to provide an acid functional polyester polyol. Trimellitic anhydride has one anhydride group and lowers the average number of hydroxyl groups per molecule, or the hydroxyl functionality, significantly. The resulting polyurethanes have low molecular weight and produce coatings which are too soft and weak to be of commercial use. In addition, the coatings exhibit yellowing from ultraviolet rays upon exposure to sunlight.
A commercial improvement over DMPA is the use of LEXOREZ.RTM. 1405-65 for forming polyurethanes. LEXOREZ.RTM. 1405-65 is available from Inolex Chemical Company of Philadelphia, Pa. and is a polyester polyol which has a typical acid valve of 50, a typical hydroxyl number of 65 and a hydroxyl functionality less than 2. The polyol is formed from esterified polyols, polyacids and aromatic anhydrides. The resulting polyol, while better than DMPA in polyurethane formation, is not appropriate for many applications due to its low hydroxyl functionality which limits the ability to build the molecular weight of the polyurethane and makes it difficult to synthesize hard, strong polyurethane materials. As such, polyurethane coatings formed from this product are usually too soft. In addition, polyurethanes formed from LEXOREZ.RTM. 1405-65 tend to show a distinct yellowing from ultraviolet rays upon exposure to sunlight, sometimes as quickly as after only one day of exposure. The yellowing is suspected to arise as the result of a chromophore which is affected by sunlight exposure.
As such, there is a need in the art for a polyol which provides a polyurethane having good mechanical properties and sufficient phase separation for hard segment formation. Further, there is a need for a polyol which does not exhibit yellowing in the manner of prior art polyols upon exposure to ultraviolet rays.
Polyurethane properties can be greatly improved by forming ionomeric polyurethanes. Such ionomers can be formed from carboxylic acid functionalized polyols or polyurethanes by neutralizing the acid groups with a base. By incorporating ionic groups in the urethane backbone, the polymer can be made more polar which can improve solvent resistance, and, depending upon the base used, can lead to either hydrophilic or hydrophobic properties. The ionic groups become sites for inter- and intramolecular attractions which increase the cohesive energy of the polymer. The bonds act in a manner similar to a covalent crosslinking but are capable of disassociation and reassociation subject to the application of stress, whereas a covalent bond would break under stress and cannot reform. Such ionomeric polyurethanes are particularly useful for forming hydrophilic polyurethane foams. Typical hydrophilic foams are known in the industry and are manufactured using polyols having a high percentage of alkylene oxide, such as ethylene oxide, with a low isocyanate index or with special hydrophilic additives. Typically, such foams become unprocessable at high levels of hydrophilicity, and are limited to moderate hydrophilic properties.
Ionomer polyurethane foams can be formed with higher levels of water than standard foams, and provide foams which are so hydrophilic they decompose quickly in water. By controlling and reducing the degree of hydrophilicity from this maximum, foams can be made hydrophilic with good wet strength. There is a need in the art for suitable hydrophilic polyurethane foams having improved hydrophilic properties and strength and for water borne polyurethanes for use in coatings, adhesives, sealants and the like which are formed from ionomeric polyurethanes, which exhibit good mechanical properties, and are easy and inexpensive to use in current polyurethane manufacturing processes. There is also a need in the art for a water borne polyurethane which is non-yellowing from ultraviolet rays upon exposure to sunlight.